The majority of refrigeration, air conditioning and heat pump equipment employed today uses the subcritical vapour compression cycle to transfer heat from a colder region to a hotter region via the low pressure evaporation and high pressure condensation of a refrigerant fluid. The refrigerants in current commercial use are either: single fluids, for example 1, 1,1,2-tetrafluoroethane (R-134a); azeotropic mixtures, for example the azeotropic mixture of 1,1,1-trifluoroethane (R-143a) and pentafluoroethane (R-125) sold as R-507; or non-azeotropic (“zeotropic”) mixtures in which the temperature glide, i.e. the difference between the bubble and dew point temperatures of the refrigerant mixture at atmospheric pressure, is relatively narrow, typically less than about 10° C. Examples of the latter include the 50:50 w/w mixture of difluoromethane (R-32) and R-125 sold as R-410A (glide less than 0.1° C.) and the 23:25:52 w/w mixture of R-32, R-125 and R-134a sold as R-407C (glide 7° C.). It has been found and accepted that all three types of refrigerant can be handled with the same kind of servicing equipment and generally used in a similar manner, with the proviso that mixed refrigerants should be transferred from the cylinder in which they are stored to the equipment as a liquid to preserve the composition.
The fluorocarbon based refrigerants in current use have relatively high Global Warming Potentials (GWP), expressed in terms of their ability to trap heat in the atmosphere on a relative scale where 1 kg of CO2 is taken as having a GWP of 1. For example, using the internationally accepted IPCC AR-4 (Intergovernmental Panel on Climate Change—Fourth Assessment Report) values of halocarbon GWPs; R-134a has a GWP of 1430; R-410A has a GWP of 2088; and R-407C has a GWP of 1774.
The environmental impact on the climate of operating a refrigeration, air conditioning or heat pump system can be expressed as an equivalent emission of CO2 greenhouse gas resulting from operation or servicing of the unit. The total equivalent warming impact (TEWI) of a system is a measure of this emission expressed as a sum of direct emission—the equivalent amount of CO2 represented by leakage of a refrigerant having a GWP—and the indirect effect, namely the CO2 emitted as a consequence of generating mechanical or electrical power to operate the unit, either by direct combustion of fuel (e.g. for automobile air conditioning) or by combustion of fuel in a power station to generate electricity.
Reduction of the GWP of the refrigerants used in such vapour compression technology therefore offers a means of reducing the overall TEWI of the technology. This is already mandated in Europe by the European Union MAC Directive for the specific application of refrigerants in automotive air conditioning. Refrigerant R-134a is currently used worldwide in automotive air conditioning systems. Under the MAC directive its use in new cars will progressively be replaced by a fluid or fluid compositions having a GWP of less than 150 over the period 2011-2017.
In the search for alternatives to the currently used refrigerant fluids, it is evident that in at least some applications those fluids will not easily be replaced by single fluids having comparable refrigerant properties without other complications arising from the other properties of the replacement fluids. The most significant of these is flammability—many molecules otherwise suitable for refrigeration applications and having low GWP are flammable.
For example, chlorodifluoromethane (R-22) (an ozone depleting fluid with a GWP of ˜1800) could be replaced from a technical standpoint by propane (GWP of 3), as the key physical properties relevant to refrigeration performance (principally boiling point and vapour pressure) are similar for the molecules. However, propane is extremely flammable whereas R-22 is non-flammable, and its flammability would preclude it from use in many applications serviced by R-22.
Therefore, it is potentially necessary in the search for new refrigerants having lower GWP to contemplate mixing refrigerant components to form a refrigerant blend having the desired balance of properties including: good refrigeration performance in the application, low flammability, low toxicity, low GWP and technical suitability for the application demands.
One class of mixtures, not currently in widespread use, which may offer promise is that of so-called wide-glide zeotropic refrigerant mixtures. These are mixtures having a temperature glide significantly larger than those exhibited by the currently used zeotropic refrigerants, e.g. a temperature glide of greater than 10° C. or perhaps greater than 15° C. Non limiting examples of such mixtures include:                Mixtures of carbon dioxide with hydrofluoroalkene fluids, such as 2,3,3,3-tetrafluoropropene (R-1234yf), 1,3,3,3-tetrafluoropropene (R-1234ze—E or Z isomers or mixtures thereof), 3,3,3-trifluoropropene (R-1243zf), 1,2,3,3,3-pentafluoropropene (R-1225ye—E or Z isomers or blends thereof), 1-chloro trifluoropropene (R-1233zd—E or Z isomers), 2-chloro 3,3,3-trifluoropropene (R-1233xf), hexafluorobutene (R-1336—all isomers), octafluoropentene (R-1438—all isomers and especially 1,1,1,4,4,5,5,5-octafluoro-2-pentene (R-1438m/z)), nonafluoropentene (R-1429—all isomers and especially 1,1,1,2,4,4,5,5,5-nonafluoro-2-pentene (R-1429myz) and 1,1,1,3,4,4,5,5,5-nonafluoro-2-pentene (R-1429mzy) and the like.        Mixtures as above additionally containing other saturated fluorocarbon refrigerant compounds, such as R-125, R-32, R-134a, fluoroethane (R-161), R-143a, 1,1,1-trifluoropropane (R-263fb), 1,1-difluoroethane (R-152a) and 1,1,1,2,3,3,3-heptafluoroethane (R-227ea), or hydrocarbons, such as propane, propylene, n-butane, isobutane, or dimethyl ether.        Mixtures of low boiling point fluorocarbon or hydrocarbon fluids with hydrofluorocarbon or hydrochlorofluorocarbon fluids having significantly higher boiling points, for example mixtures comprising R-32 (boiling point −51° C.) with other halogenated refrigerant fluids having boding points higher than R-134a.        
For zeotropic refrigerant blends, compositional shifts arise as the liquid blend is discharged from the container in which it is stored. As the liquid refrigerant blend is removed from the container, the space above the liquid refrigerant increases allowing it to accommodate more vapour. However, for zeotropic blends, the more volatile refrigerant component evaporates preferentially, so that the vapour space above the liquid becomes occupied with a vapour composition that becomes progressively enriched in the more volatile component. Correspondingly, the liquid refrigerant that remains in the storage container becomes gradually depleted in the more volatile component as more and more of the liquid is removed from the container. This behaviour is known as composition shift.
In automotive air conditioning systems, SAE standard J1658 requires that the performance of a mixed refrigerant be evaluated across the range of compositions that develop during depletion of a cylinder containing the mixed refrigerant from full to empty. This standard requires the change in cooling capacity resulting from the compositional change to be less than 5% over the range.
The zeotropic refrigerant mixtures in use today, such as R-407C, have relatively narrow temperature glides. These mixtures are conventionally manufactured by a batch wise blending operation in which the components are added to a bulk blending tank in reverse order of volatility. Optionally, recirculation of the liquid contents of the tank is used to ensure good mixing of the components. On completion of the blending process the tank contains a liquid refrigerant phase and a vapour refrigerant phase. The compositions of the two phases are different for a zeotropic mixture, with the vapour phase containing more of the more volatile components than does the liquid phase. The relative amounts of each component added in this batch wise blending process are selected to ensure that the liquid phase meets the target composition specification.
Once the bulk blending operation is complete, the refrigerant is transferred into smaller cylinders or tanks by withdrawal of liquid from the bulk blending tank. As explained supra, as this withdrawal occurs, the proportion of the more volatile species in the liquid phase is progressively depleted. This effect is illustrated in European patent EP-B-0,767,348 assigned to Daikin Industries, which deals with the handling and packing of R-407C compositions. For R-407C, the patent attends to the problem of composition shift by initially placing a composition enriched in R-32 in the blending vessel so as to ensure that throughout the process of liquid removal, the content of R-32 in the liquid stays within specification, despite the preferential evaporation of the more volatile R-32 into the increasing vapour space volume as liquid is removed.
This technique may, however, be insufficient to allow zeotropic mixtures having large glides, such as the zeotropic mixture of CO2, R-134a and R-1234ze(E), to be handled so that the liquid composition delivered from a blending vessel stays within specification.
Furthermore, when a zeotropic refrigerant blend is registered with the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE), it will have tolerance limits for each component in the blend. For example, for R-407C, there is a ±2 weight % tolerance on each component in the blend. These tolerance limits must be observed during the subsequent use of the blend. This need to observe and control composition specification can present a problem with wide-glide zeotropic refrigerant mixtures, as the compositional shift that results as liquid is removed from the cylinder can eventually take the liquid refrigerant out of specification.